The purpose of the proposed project are to characterize the influence of infant, maternal and tester behavior on infants' motor performance and to address theoretical issues about the relationship between infant affect and performance vs. competence. Specifically, the project has three aims: 1) to devise methods for characterizing behavioral aspects of infant motor performance, 2) to describe the relationship between behavior and motor test scores, and 3) to test hypotheses about performance vs. competence in infant motor function. Accomplishing the first aim (instrument development) utilizes two different methodologic approaches--quantitative coding of specific behaviors and qualitative rating scales. The project's second aim, describing the relationship between behavior and motor test scores, will entail the analysis of videotapes of the motor test of the Bayley Scales of Infant Development of 191 12- to 23-month-old infants. Summary measures of infant affect, task orientation, interaction with adults, and quality of movement will be created from the behavior codes and ratings. Behavioral profiles of infants whose motor scores are high, average, or low will be used to describe the relationship between behavior and motor test scores. The final specific aim of the project is to test two hypotheses about performance vs. competence in motor function. The first hypothesis is that sociable infants perform better on developmental assessments due to their willingness to interact and cooperate with the tester, rather than to their greater abilities; the second maintains that such infants are actually more competent, because they have invited and received more stimulation from caregivers. These alternatives can be partly addressed in the proposed project, because other sources of information about study infants' motor abilities are available. These sources include a videotaped play period that provides an opportunity to assess infants' motor function when alone with the mother and a second motor test, administered after one week, that document the effects of familiarization with tester and test situation. The proposed study with thus develop new methods for assessing previously uncharacterized influences on motor performance, assess the relationship between motor skills and a variety of other factors (birth, nutritional status, family background, and the home environment), and address theoretical issues about the relationship between sociability and developmental status and its assessment in infancy.